I'm studying Waves in Cold plasmas right now, but I guess my question is generalizable. It's about the 4th Maxwell Equation in polarizable / conductive media:
∇×H=1c∂D∂t+4πcJ
Now from our electrodynamics lectures we know that D=ϵE with ϵ being the dielectric tensor, but also the crucial modellization of Ohm's law for conductive media J=σE is well within our memory. So my first reflex would now be to just plug in everything and obtain an equation for E in Fourier-space and see where I get to. However this seems never to happen in the literature, and ppl as Stix (Waves in Plasmas, Chap1) or Jackson establish the relationship ϵ=1−4πσiω.
This is now what confuses me severly:
D arises from the argumentation that if we have bound charges in a medium, they will linearly answer with a polarisation P so that D=E+4πP. The conductivity involves a (to be specified) steady-state modell of free charges in response to a electric field.
Although fundamentally different views, the bound and free modell seem to be connected by ϵ=1−4πσiω, but I'd be glad for some physical motivation. In the end (for cold plasmas at least), we always use J=nmv and v from the equation of motion to obtain a relationship between J and E, thus obtaining σ. But there are also sources which do stuff with the dielectric tensor which I dont really understand (like Padmanabhan, Theoretical Astrophysics Vol I., Chap 9.5).
So concluding, my questions would now be the following:
- Naively plugging in D=ϵE and J=σE into Maxwell 4 seems to be wrong, also due to the physical reasonings behind those modells. We can't have both in the equation. True/False?
- Why can such a relation as ϵ=1−4πσiω exist between a static and a dynamic modell? Or am I mislead here and the iω hints at a dynamic origin for ϵ too?
Thanks in advance, for anyone taking the time!
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